(1) Field of the Invention
The present invention relates to a compatible optical disk. Namely, the present invention relates to a rewritable optical disk compatible with a compact disk (CD) used exclusively for reproduction. More particularly, the present invention relates to an optical disk in which information can be reproduced by using a commercially available standard CD player and writing, rewriting, and erasure can be performed.
(2) Description of the Related Art
Compact disks are industrially valuable because they enable users to easily utilize large information (voices, images and the like) capacities normally used exclusively for reproduction, at a low cost. Nevertheless, where writing, rewriting, or a partial change of information is desired, a new original disk must be prepared, and this incurs much expense and time. Accordingly, if a CD used exclusively for reproduction can be made writable or rewritable, and a conventional player normally used exclusively for reproduction can be used for a reproduction of information stored in this disk, a valuable contribution to society can be made since CD players already widely available can be effectively utilized.
Rewritable optical disks and additional write type optical disks in which information can be only recorded have been intensively investigated, and disks of this type have been marketed.
In disks of this type, however, to increase the recording sensitivity, the absorption of light is enhanced, and thus the reflectivity is kept to a low level, and accordingly, information cannot be read out by commercially available CD players. Namely, a reflectivity of at least 70% is stipulated as the standard value for a CD used exclusively for reproduction, and players are constructed to comply with this standard, but the reflectivity of rewritable optical disks or additional write type optical disks is generally as low as 15 to 50%. Furthermore, additional write type optical disks are usually perforation type, i.e., holes are formed in the recorded areas, and accordingly, the reflectivity is almost 0% in the recorded areas. Therefore, the reflectivity in the recorded area is lower than the reflectivity before the recording, and for this reason also, reproduction by CD players is impossible.
To provide a compatible writable or rewritable optical disk in which reproduction is possible by a standard CD player, the reflectivity of the disk must be increased, but this increase of the reflectivity results in a reduction of the recording sensitivity.
Furthermore, to perform the recording, the recording beams must be positioned in the form of tracks at a predetermined part of the disk. If information is already recorded, it is possible for information tracks to be traced, but if the disk is unrecorded and has no specific structure, it is very difficult to position tracks for the recording, and a guidance structure is required.
Usually, a groove is used as the guidance structure, and the shape of the guide groove must satisfy certain conditions. For example, where a signal is recorded along a trough portion, a condition represented by the following inequality must be satisfied: ##EQU1##
If this condition (1) is not satisfied, for a CD player wherein a 3-beam tracking system is adopted, light beams trace the crest portion and the signal cannot be reproduced. To satisfy the condition (1), the width of the trough portion must be narrowed, but if the width of the trough portion is narrowed, the amplitude of the signal is reduced and the CN ratio (carrier/noise ratio) is reduced.